Method for reducing radioactivity of oilfield tubular goods contaminated with radioactive scale

ABSTRACT

Alkaline earth metal scales, especially barium sulfate scale deposits are removed from oilfield pipe and other tubular goods with a scale-removing composition comprising an aqueous alkaline solution having a pH of about 8 to about 14, a polyaminopolycarboxylic acid, preferably EDTA or DTPA and a catalyst or synergist comprising oxalate anion. When the scale-removing solution is contacted with a surface containing a scale deposit, substantially more scale is dissolved at a faster rate than previously possible.

This is a division of copending application Ser. No. 369,897, filed onJune. 22, 1989, now U.S. Pat. No. 4,980,077, which is a continuation inpart of Ser. No. 332,147, filed Apr. 3, 1989.

FIELD OF THE INVENTION

This invention relates to compositions which are effective forsolubilizing and removing scale, particularly strontium and bariumsulfate scale, from surfaces with scale deposits on them. It isparticularly useful for the removal of such scale from oilfieldequipment including downhole pipe, tubing and casing as well assubterranean formations. It is also applicable to the removal of thesescale deposits from other equipment such as boilers and heat exchangers.

BACKGROUND OF THE INVENTION

Many waters contain alkaline earth metal cations, such as barium,strontium, calcium and magnesium, and anions, such as sulfate,bicarbonate, carbonate, phosphate, and fluoride. When combinations ofthese anions and cations are present in concentrations which exceed thesolubility product of the various species which may be formed,precipitates form until the respective solubility products are no longerexceeded. For example, when the concentrations of the barium and sulfateions exceed the solubility product of barium sulfate, a solid phase ofbarium sulfate will form as a precipitate. Solubility products areexceeded for various reasons, such as evaporation of the water phase,change in pH, pressure or temperature and the introduction of additionalions which can form insoluble compounds with the ions already present inthe solution.

As these reaction products precipitate on the surfaces of thewater-carrying or water-containing system, they form adherent depositsor scale. Scale may prevent effective heat transfer, interfere withfluid flow, facilitate corrosive processes, or harbor bacteria. Scale isan expensive problem in many industrial water systems, in productionsystems for oil and gas, in pulp and paper mill systems, and in othersystems, causing delays and shutdowns for cleaning and removal.

Barium and strontium sulfate scale deposits present a unique andparticularly intractable problem. Under most conditions, these sulfatesare considerably less soluble in all solvents than any of the othercommonly encountered scale-form ng compounds, as shown by thecomparative solubilitie given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Comparative Solubilities, 25° C. in Water.                             Scale           Solubility, mg./l.                                            ______________________________________                                        Gypsum          2080.0                                                        Strontium sulfate                                                                              140.0                                                        Calcium Carbonate                                                                              14.0                                                         Barium sulfate    2.3                                                         ______________________________________                                    

It is generally acknowledged that barium sulfate scale is extremelydifficult to remove chemically, especially within reasonably shortperiods of time: the solvents which have been found to work generallytake a long time to reach an equilibrium concentration of dissolvedbarium sulfate, which itself is usually of a relatively low order.Consequently, barium sulfate must be removed mechanically or theequipment, e.g. pipes, etc., containing the deposit must be discarded.

The incidence of barium sulfate scale is worldwide, and it occursprincipally in systems handling subsurface waters. Because of this, thebarium sulfate scale problem is of particular concern to the petroleumindustry as water is generally produced with petroleum and as time goeson, more petroleum is produced by the waterflooding method of secondaryrecovery, implying even greater volumes of produced water. The scale mayoccur in many different places, including production tubing, well boreperforations, the area near the well bore, gathering lines, meters,valves and in other production equipment. Barium sulfate scale may alsoform within subterranean formations such as in disposal wells. Scalesand deposits can be formed to such an extent that the permeability ofthe formation is impaired resulting in lower flow rates, higher pumppressures, and ultimately abandonment of the well.

Barium sulfate scale is particularly troublesome when sulphate-richseawater is used as an injection fluid in oil wells whose formationwater is rich in barium ions. This particular aspect of the barium scaleproblem is severe in some U.S. oil fields as well as some older NorthSea oil fields. Scaling of this nature is also expected to occur duringadvanced production stages in other North Sea fields particularly afterseawater breakthrough has taken place.

Another problem associated with the formation of barium and strontiumsulfate scales is that radium, another member of the alkaline earthgroup of metals tends to be deposited at the same time so that theequipment becomes radioactive, and may eventually have to becomeunusable for safety reasons alone. At present, a considerable amount ofoilfield tubular goods are in this condition and cannot be readilyrestored to usable condition because of the difficulty of removing theradioactive scale.

Various proposals have been made in the past for removing barium sulfatescale chemically. Most of these processes have utilised chelating orcomplexing agents, principally the polyaminopolycarboxylic acids such asethylenediaminetetraacetic acid(EDTA) or diethylenetriaminepentaaceticacid(DTPA).

U.S Pat. No. 2877848(Case) discloses the use of EDTA in combination withvarious surfactants for this purpose. U.S. Pat. No. 3660287 (Quattrini)discloses the use of EDTA and DTPA in the presence of carbonate ion atrelatively neutral pH (6.5-9.5) and U.S. Pat. No. 4,708,805(D'Muhala)discloses a process for the removal of barium sulfate scale bysequestration using an aqueous solution of citric acid, a polycarboxylicacid such as carbazic acid, and an alkylene-polyaminopolycarboxylic acidsuch as EDTA or DTPA. The preferred aqueous sequestering solutions havea pH in the range of about 9.5 to about 14, provided by a base such aspotassium hydroxide or potassium carbonate.

Another approach which has recently been made is to use a polyether incombination with the aminopolycarboxylic acid. U.S. Pat. No.4,190,462(deJong) discloses that barium sulfate scale can be removedfrom remote locations extending into a subterranean earth formation bycontacting the scale with an aqueous solution consisting essentially ofwater, a monovalent cation salt of a monocyclic macroyclic polyaminecontaining at least two nitrogen-linked carboxymethyl groups and enoughmonovalent basic compound to provide a solution pH of about 8. Similardisclosures are to be found in U.S. Pat. Nos. 4,215,000 and 4,288,333.These polyether materials have, however, the disadvantage of beingcostly which is a severe drawback for oilfield use where cost is a majorfactor.

Although many of these known compositions will remove scale, the rate ofdissolution is slow and the amount of scale dissolved is small.

SUMMARY OF THE INVENTION

We have now found a way of removing barium sulfate scale using a novelcombination of scale-removing agents. This combination is capable ofremoving scale at markedly higher speeds than prior scale-removingcompositions and is also capable of removing relatively more scale for agiven quantity of solvent. It is, moreover, relatively cheap and istherefore well suited to use in oilfield operations.

According to the present invention, barium sulfate and other sulfatescales are removed by a chemical process using a combination of apolyaminopolycarboxylic acid such as EDTA or DTPA together with anoxalate anion as a synergist or catalyst for the dissolution. The scaleis removed under alkaline conditions, preferably at pH values of atleast 10, usually 10-14, with best results being achieved at about pH12.

The concentration of synergist or catalyst is usually about 0.01 M toabout 1.0 M, preferably about 0.5 M, with similar concentrations beingappropriate for the primary chelant (the polyaminopolycarboxylic acid).Substantially improved scale dissolution rates are obtained when theaqueous solution containing the composition is at a temperature of about25.C to about 100° C. but higher temperatures are obtainable downholebecause at greater formation depths higher existing pressures will raisethe boiling point of the aqueous solution, and cosequently greater scaleremoval rates may be attained. The composition is particularly usefulfor more efficiently removing barium or strontium sulfate scale fromwells, wellstream processing equipment, pipelines and tubular goods usedto produce oil from a subterranean formation.

THE DRAWINGS

FIG. 1 is a graph which shows the rate of dissolution of barium sulfatein various solvents,

FIG. 2 is a graph which shows the effect of chelant concentration on therate of barium sulfate dissolution,

FIG. 3 is a graph which shows the effect of chelant concentration on therate of barium sulfate dissolution,

FIG. 4 is a graph which shows effect of temperature on the rate ofbarium sulfate dissolution,

FIG. 5 is a graph which shows the respective rates of dissolution ofvarious sulfate species on a chelant-containing solvent,

FIG. 6 is a graph which shows the respective rates of dissolution ofvarious barium sulfate species in a chelant-containing solvent,

FIG. 7 is a graph which shows the low residual rates of

radioactivity which may be achieved for contaminated

oilfield pipe by use of the present scale removal process.

DETAILED DESCRIPTION

According to the present invention, alkaline earth metal sulfate scales,especially barium sulfate scale, are removed by the use of a combinationof chemical scale-removing agents. The method is particularly useful forthe removal of such scale from oilfield equipment used to bring oiland/or water from subterranean formations to the surface. The methodmay, however, also be used to remove scale from the formationsthemselves, especially in the regions surrounding production andinjection wells, as mentioned above. The method may also be used toremove scale from above- ground equipment both in the oilfield andelsewhere, for example, from boilers and heat exchangers and otherequipment exposed to scale-forming conditions.

The scale itself is usually in the form of an adherent deposit of thescale-forming mineral on metal surfaces which have been exposed to thewater containing the scale-forming components. These components comprisealkaline earth metals including calcium, strontium and barium, togetherwith variable amounts of radium, depending upon the origin of thewaters. As noted above, barium sulfate scale is particularly difficultto remove by existing chemical methods in view of its very lowsolubility.

The present scale removal is effected with an aqueous solvent whichcomprises a polyaminopolycarboxylic acid such as EDTA or DTPA as achelant o chelating agent which is intended to form a stable complexwith the cation of the alkaline earth scale-forming material. Of thesechelants, DTPA is the preferred species since it forms the most solublecomplexes at greatest reaction rate. EDTA may be used but is somewhatless favorable as will be shown below. The chelant may be added to thesolvent in the acid form or, alternatively, as a salt of the acid,preferably the potassium salt. In any event the alkaline conditions usedin the scale removal process will convert the free acid to the salt.

The concentration of the chelant in the solvent should normally be atleast 0.1M in order to achieve acceptable degree of scale removal.Chelant concentrations in excess of 1.0 M are usually not necessary andconcentrations from about 0.3M up to about 0.6M will normally give goodresults; although higher concentrations of chelant may be used, there isgenerally no advantage to doing so because the efficiency of the chelantutilisation will be lower at excess chelant concentrations. Thiseconomic penalty is particularly notable in oilfield operations wherelarge volumes of solvent may be used, especially in formation scaleremoval treatment.

The concentration of the catalyst or synergist in the aqueous solventwill be of a similar order: thus, the amount of the oxalate anion in thesolvent should normally be at least 01.M in order to achieve aperceptible increase in the efficiency of the scale removal, andconcentrations from about 0.3M up to about 0.6M will give good results.Although higher concentrations of the oxalate e.g. above 1.0 M may beused, there is generally no advantage to doing so because the efficiencyof the process will be lower at excess catalyst concentrations. Again,this economic penalty is particularly notable in oilfield operations.

As with the chelant, the oxalate may be added as the free acid or thesalt, preferably the potassium salt. If the free acid is used, additionof the potassium base to provide the requisite solution pH will convertthe acid to the salt form under the conditions of use.

The scale removal is effected under alkaline conditions preferably at pHvalues of from about 8.0 to about 14.0, with optimum values being fromabout 11 to 13, preferably about 12.

The preferred solvents comprise about 0.1 to about 1.0 M ofethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaaceticacid (DTPA), or salts of these acids, as a chelant. In addition, theoxalate catalyst is added to the aqueous solution in about 0.01 to about1.0, preferably about up to 0.5 M. The pH of the solvent is thenadjusted by the addition of a base to the desired value, preferably toabout pH 12. We have found that it is important to avoid the use ofsodium cations when operating at high pH values, above pH 8, andinstead, to use potassium or, alternatively, cesium as the cation of thescale-removing agent. Potassium is preferred for economy as well asavailability. Thus, the normal course of making up the solvent will beto dissolve the chelant and the oxalic acid (or potassium oxalate) inthe water to the desired concentration, after which a potassium base ,usually potassium hydroxide is added to bring the pH to the desiredvalue of about 12. This aqueous composition can be used to remove scalefrom the equipment, or alternatively, pumped into the subterraneanformation when it is the formation which is to be subjected todescaling.

The mode of operation of the oxalate synergist or catalyst is notpresently understood. While not desiring to be bound to a particulartheory concerning the actual mechanism of its activity in converting ordissolving the scale, it is believed that adsorption of the synergist orcatalyst on the barium sulfate surface may modify the surface crystalstructure in such a way that the barium in the modified crystal iseasily removed by the chelating agent.

The aqueous solution containing the composition can be directed down awellbore to remove barium sulfate scale which has fouled the tubularequipment e.g. piping, casing etc., and passage ways. Prior to beingdirected into the wellbore, the composition may be heated to atemperature between about 25° C. to about 100° C., although thetemperatures prevailing downhole may make pre-heating unnecessary. Oncewithin the tubular goods and the passageways requiring treatment, thecomposition is allowed to remain there for about ten minutes to about 7hours. After remaining in contact with the equipment for the desiredtime, the composition containing the dissolved scale is produced to thesurface and may be disposed of as required, possibly by re-injectioninto the subsurface formation. This procedure can be repeated as oftenas required to remove scale from the equipment.

In one procedure for circulating the solvent through the tubular goodsin the well the solvent is pumped down through the production tube andreturned to the surface through the annular space between the productiontubes and the casing (or vice versa). Also, the cleaning solution may bepumped down through the production tubing and into the formation,thereby cleaning the well, including the well casing, and the formationpore space by dissolving barium sulfate present as it flows over andalong the surfaces that need cleaning. The spent composition containingthe dissolved, complexed barium together with any other alkaline earthmetal cations which may have been present in the scale, especiallyradium, can be subsequently returned to the surface, for example, bydisplacement or entrainment with the fluids that are produced throughthe well after the cleaning operation. In an alternative manner, thecleaning solution may be applied batchwise fashion, for example, byflowing the solution into the well and optionally into the pore spacesof the adjacent earth formation and there keeping the solution incontact in non-flowing condition with the surfaces that are covered withbarium sulfate scale, for a period of time sufficient to dissolve thescale.

In order to demonstrate the barium sulfate scale-dissolving capacitiesof the composition, several aqueous solutions have been tested inlaboratory tests the results of which are described in the discussionswhich follow. The experiments described below were, except as notedbelow, carried out in a cylindrical glass vessel having a height of 10cm and an internal diameter of 7.5 cm. Barium sulfate or, whenapplicable, other sulfates or solid scale components, were agitated withthe selected solvents and the rates of dissolution and final dissolvedconcentrations determined. The results are reported graphically in theFIGS.

As shown in FIG. 1, various concentrations of EDTA, DTPA, and DTPA withoxalate were compared at 25° C. and 100° C. The results demonstrate thatthe DTPA/oxalate combination complexes more barium sulfate than DTPAalone and that DTPA is more effective than EDTA at both temperatures.Furthermore, when the oxalate is present with the DTPA, the equilibriumconcentration of dissolved barium sulfate is reached far more quicklythan with either the EDTA the DTPA, which have not attained equilibriumafter 7 hours at the termination of the experiment.

The dissolution of the barium sulfate or other scale in the solvent isinfluenced by the amount of chelant used. The effect of varying the DTPAconcentration (at 100° C.) is shown in FIG. 2, at chelant concentrationsfrom 0.1M to 0.6M. Increased DTPA concentration causes an increase inthe rate of barium sulfate dissolution and the amount of barium sulfateheld in the solvent. It should be noted in particular that the finalequilibrium concentration of barium sulfate is 60 g./1. which is far inexcess of the solubility in water alone.

The amount of oxalate catalyst utilized in combination with DTPA is notcritical within the limits described above. This is illustrated in FIG.3 which shows that all concentrations of oxalate catalyst contribute tothe dissolution of 80 to 90 percent of the saturation level of bariumsulfate within ten (10) minutes of contact. Thus, as demonstrated byFIG. 3, the fast rate of dissolution is a significant feature of thepresent scale removal technique. In practical applications of themethod, therefore, contact times of less then about 4 hours e.g. 1 or 2hours, may be sufficient, depending on the scale thickness. Anothersignificant feature of the technique is the high equilibrium(saturation) levels of dissolved barium, strontium and calcium sulfatescales which are obtained in the aqueous solution, making the processparticularly efficient in terms of solvent utilisation..

FIG. 4 shows that the rate of dissolution of the barium sulfate scale isrelated to temperature, with faster rates of dissolution being attainedat the higher temperature (100° C.)

FIG. 5 shows the results of a batch test carried with scale materialremoved from field tubing similar to that used in a continuous flow looptest in which the test solution was circulated. The samples of scalematerial were added to the solvent (0.5M DTPA,0.5M oxalate pH=12, 100°C.) in a concentration equivalent to 60 g./1. of scale. Theconcentrations of the different species dissolved in the solvent atdifferent times were determined. The results of the batch tests areshown in the figure and indicate that in addition to the dissolution ofthe barium, the strontium sulfate also reaches equilibrium concentrationin a very short time. The results from the flow loop tests are similarbut with much lower final concentrations.

FIG. 6 shows that the scale removal process is effective both withbarium sulfate in the powder form and also with actual pipe scale/tarmixtures and with barite ore (BaSO₄).

FIG. 7 shows that the present scale removal technique is very effectivefor lowering residual radioactivity of pipe contaminated withradium-containing barium sulfate scale. As noted above, radium isfrequently precipitated with barium in scale with the result that scaledpipe is often radioactive to the point that it cannot safely be used. Acontinuous flow loop test was used to remove the scale from pipe whichwas similar to that used with FIG. 5 and the radioactivity wasdetermined at successive times during the test. As shown in the FIG.,the activity was reduced to an acceptable level after three hourswithout further treatment. It appears, however, that some residualactivity arises from lead and other radio-isotopes which are notdissolved in the solvent (see FIG. 5); these isotopes are decay productsof radium and have originally been incorporated in the scale with thebarium and the radium sulfates. Although they are not removed chemicallyby the present scale removal technique, the dissolution of the bariumscale together with the other alkaline earth metal sulfates enabledthese other components of the scale to be removed by simple abrasion.For FIG. 7, the descaled pipe was scrubbed with a soft bristle brushusing a detergent/water scrub solution. The result was to reduce theresidual activity level to a very low value, below the appropriateregulatory standards. Thus, by using the present chemical scale removaltechnique in combination with a simple mechanical removal of loose,non-adherent material, previously radioactive pipe may quickly andreadily be restored to useful, safe condition.

Distilled water was used in the majority of the above tests (except thecontinuous flow loop tests) for determination of the rate of bariumsulfate dissolution and saturation. Some tests were run with Dallas citytap water and synthetic seawater. A minor decrease in efficiency wasobserved with tap water. About a 20 percent decrease in efficiency wasobserved when seawater was used. This was expected, since seawater hasinterfering ions, e.g. calcium and magnesium. These interfering ionscomplex with the chelating agent, either DTPA or EDTA, and reduce theoverall dissolving power. Additionally, it has been determined thathalide ions have a negative effect on dissolving power as a function ofthe size of the halide ion. Dissolution rate is increased as the halideion size is reduced and the charge density is increased, i.e. in theorder of iodide, bromide, chloride and fluoride. Fluoride ion enhancesthe effect of EDTA-based solvents, but not DTPA: fluoride inhibits mostDTPA/catalyst solvents.

As noted above, the effect of cations is also very important to thesuccess of the scale solvent, especially when added with the sizableportion of caustic required to adjust the pH to 12. Dissolution isenhanced as the size of the cation is increased, i.e. lithium, sodium,potassium and cesium. Lithium and sodium hydroxides in the presence ofEDTA, or DTPA, and catalysts are not soluble at a pH of 12, the optimumvalue. Cesium is too difficult to obtain, both in quantity and price.Therefore, potassium hydroxide, in the form of caustic potash, is the pHadjusting reagent of choice.

One example of a preferred aqueous solvent which can be used comprises0.5 M DTPA and 0.3 M oxalic acid adjusted to a pH of 12 with potassiumhydroxide.

We claim:
 1. A method for reducing the radioactivity of oilfield tubulargoods contaminated with radioactive, radium-containing scale ofdeposited alkaline earth metal sulfates, comprising contacting the scaleon the tubular goods with an aqueous composition for dissolving alkalineearth sulfate scale comprising an aqueous alkaline solution having a pHof from about 8 to about 14, from about 0.1M to about 1.0M of achelating agent comprising a polyaminopolycarboxylic acid or a salt ofsuch an acid, and from about 0.1M to about 1.0M of oxalate anion toimprove the scale removing characteristics of the composition, for atime sufficient to remove the scale from the tubular goods.
 2. Themethod as recited in claim 1 where the chelating agent comprises DTPAand the pH is from 11 to 13 provided by potassium hydroxide.
 3. Themethod as recited in claim 1 where the tubular goods are subjected tomechanical abrasion after removal of the scale by the solution.
 4. Themethod as recited in claim 1 where the scale is contacted with thesolution at a temperature of from 25° C. to 100° C.
 5. The method asrecited in claim 1 where the scale comprises barium sulfate.